In recent years, fuel cell systems have been developed and commercialized as distributed power generation systems. An organic compound containing carbon and hydrogen is supplied as a raw material to the fuel cell system. In the fuel cell system, for example, a fuel cell reforms therein the raw material to generate a hydrogen-containing reformed gas, or a reformer provided outside the fuel cell reforms the raw material to generate the reformed gas. When the reformed gas is generated as above, the fuel cell can utilize hydrogen in the reformed gas and oxygen in air supplied from an outside to generate electricity and heat by an electric power generating reaction.
Since the fuel cell system can efficiently generate electric energy and heat energy, it has been expected as an energy supply system effective for reducing carbon dioxide that is a cause of global warming.
Examples of the raw material used in the fuel cell system include a liquefied petroleum gas (LPG), a liquefied natural gas (LNG), a city gas, a shale gas, and methane hydrate. The raw material itself contains a sulfur component, or an odorant added to the raw material contains a sulfur component. When the raw material containing the sulfur component is supplied to an anode of the fuel cell through the reformer and the like, the anode is poisoned by the sulfur component, and this deteriorates a performance of the fuel cell. Further, a reforming catalyst contained in the reformer is poisoned, and this deteriorates a reforming performance. Therefore, the raw material needs to be supplied to the reformer and the anode after the sulfur component in the raw material is reduced to a ppb order or a sub-ppb order.
Therefore, the fuel cell system includes a desulfurizer provided upstream of the reformer and having a function of reducing the sulfur component in the raw material. Examples of a method of removing the sulfur component in the raw material by the desulfurizer include: normal temperature desulfurization in which the sulfur component is physically adsorbed on a catalyst at normal temperature to be removed; and hydrodesulfurization in which the sulfur component is removed by adding hydrogen to the raw material. A hydro-desulfurizer includes a catalyst having an active temperature range that is a predetermined temperature range (for example, about 150 to 350° C.). The hydro-desulfurizer generates hydrogen sulfide from hydrogen and the sulfur component in the raw material supplied from an outside and causes the catalyst to chemically adsorb sulfur in the hydrogen sulfide.
Therefore, when desulfurizing the raw material using the hydro-desulfurizer, hydrogen and the raw material having passed through the hydro-desulfurizer after the hydro-desulfurizer has reached a predetermined temperature (for example, 150° C.) need to be supplied to the reformer and the anode of the fuel cell.
Therefore, when starting up the fuel cell system, the raw material having passed through a normal temperature desulfurizer is supplied to the reformer and the anode of the fuel cell until the hydro-desulfurizer reaches a predetermined temperature (for example, about 180° C.). Thus, the reformer and the fuel cell are warmed up. The raw material having passed through the reformer and the anode of the fuel cell or the reformed gas is combusted together with air supplied to a cathode of the fuel cell, and the hydro-desulfurizer, the reformer, and the fuel cell are heated and warmed up by heat of the combustion and heat of an exhaust gas of the combustion.
If the normal temperature desulfurizer is low in cost and can adsorb and remove the entire sulfur component in the raw material at a ppb order or a sub-ppb order, only the normal temperature desulfurizer may be used, and the hydro-desulfurizer is unnecessary. However, the normal temperature desulfurizer cannot satisfy both of these requirements. On the other hand, the hydro-desulfurizer has a high sulfur component removal performance and is low in cost but needs to be heated and warmed up.
Therefore, during the warm-up of the hydro-desulfurizer, the raw material needs to be desulfurized by normal temperature desulfurization. Thus, both the hydro-desulfurizer and the normal temperature desulfurizer are used in many cases.
One example of the fuel cell system using both the hydro-desulfurizer and the normal temperature desulfurizer is described in PTL 1.
In the fuel cell system of PTL 1, the raw material passes through the normal temperature desulfurizer during the warm-up of the reformer, the fuel cell, and the hydro-desulfurizer. After the warm-up is completed, a flow route of the raw material is switched from a route through which the raw material flows to the normal temperature desulfurizer to a route through which the raw material bypasses the normal temperature desulfurizer and flows to the hydro-desulfurizer. Then, a part of the reformed gas generated by reforming of the raw material returns as a recycled gas to a raw material supply passage provided upstream of the hydro-desulfurizer. With this, the raw material and the hydrogen can be supplied to the hydro-desulfurizer.